Connectable interbody implant

ABSTRACT

An intersomatic implant ( 1   a,    1   b ) maintains bone graft material in a receiving cavity formed in a disc in order to obtain intervertebral fusion. The implant ( 1   a,    1   b ) includes of a plurality of parts provided with means for in situ connection of two consecutive parts. Instruments ( 33 ) are used to connect these implants ( 1   a,    1   b ) and a tool assists in introducing these implants ( 1   a,    1   b ) into the cavity formed in the disc.

BACKGROUND

The present invention generally concerns the field of interbody implantswith which it is possible to obtain fusion of two adjacent vertebraewhen the disc separating them has been damaged, and more specifically,but not exclusively, it concerns the implants commonly referred to as“interbody cages” or “intersomatic cages” which are intended to receivea bone graft and to maintain the latter in place during the fusion,after partial excision of the damaged disc.

When a disc separating two vertebrae has degenerated and interbodyfusion is necessary, this can be obtained by implanting interbody cagesin cavities formed in the degenerated disc. The interbody cages areoften fitted by the anterior route, in which case it is possible only toprovide a single cage of this type, implanted in the central region ofthe disc. However, when the lumbar region is involved, an approach bythe posterior route is possible on account of the fact that the nervoussystem there is less dense than in the other regions of the spine. Thereis therefore less risk of damaging the nervous system there during thesurgical intervention. Nevertheless, the presence of the medullary canalin practice requires the use of two small sized interbody implants thatare symmetrically arranged relative to the axis of the spine. Thesebilateral implants are separate components that are not connected to oneanother. There is therefore a risk of relative displacement or expulsionof the implants, especially since an implant of small size is lessstable, in particular rotationally less stable, than an implant oflarger size.

SUMMARY

One object of the present invention is to provide a unique interbodyimplant for spinal fusion and corresponding method to facilitateimplantation of the implant.

In the case of intervertebral fusion by a posterior approach performedin the lumbar region, one object of the invention is to provide surgeonswith the possibility of using interbody implants having a high degree ofstability, while at the same time guaranteeing, as in current practice,a high level of safety during fitting of the implants.

To this end, the invention relates to an interbody implant for insertingand maintaining a bone graft in place in a receiving seat formed in adisc with a view to obtaining intervertebral fusion, characterized inthat it consists of a plurality of parts provided with means for in situconnection of two consecutive parts.

According to a first variant of the invention, the interbody implantcomprises a first part intended to be oriented in the posterior-anteriordirection of the spine, and a second part oriented substantiallyperpendicular to the first part, the parts being connected by atransition portion, the front face of the second part of the implantcomprising either a protrusion, making this implant a male implant, or areceiver for inserting and holding the protrusion, making this implant afemale implant, in order to achieve and maintain a connection betweensuch a male implant and such a female implant.

The male interbody implant can comprise, on the front face of its secondpart, a protrusion which successively comprises starting from the frontface:

-   -   a first substantially cylindrical portion having a given        diameter “D1”;    -   a second substantially cylindrical portion having a diameter        “D2” greater than “D1”;    -   and a substantially frustoconical portion whose initial diameter        is equal to “D2” and decreases as the distance from the front        face increases.

The female interbody implant can comprise, on the front face of itssecond part, a receiver delimited externally by elastic tongues whichfinish in a bulge.

The above-mentioned male interbody implant and female interbody implantmay form a unit, the shape and the dimensions of the tongues of thefemale implant and of their bulges making them able to cooperate withthe protrusion of the male implant in order to achieve and maintain aconnection of the two implants.

The male interbody implant can comprise, on the front face of its secondpart, a protrusion whose general external shape is that of a cylinderportion whose cross section has a circumference which extends over anangle greater than 180° in such a way as to define two receiving seatsin the area of its zones of connection with the front face.

The female interbody implant can comprise, on the front face of itssecond part, two elastic tabs, which between them define a receiver.

The above-mentioned male interbody implant and female interbody implantmay form a unit, the shape and the dimensions of the elastic tabs of thefemale implant making them able to permit insertion of the protrusion ofthe male implant into the receiver, the ends of the elastic tabs beinginserted into the receiving seats in order to achieve and maintain aconnection of the two implants.

The invention also relates to an instrument for in situ connection of amale interbody implant and a female interbody implant of the abovetypes, characterized in that it comprises two rods which are eachequipped with means for fixing to the front face of the first part of animplant, and means for moving the rods towards one another while holdingthem in parallel positions.

The means for moving the rods towards one another can comprise:

-   -   two tubes in which the rods can be inserted and blocked;    -   a first transverse rod connecting the front end of the first        tube to the rear end of the second tube, and a second transverse        rod connecting the front end of the second tube to the rear end        of the first tube, the transverse rods intersecting in the area        of an articulation permitting their rotation about a common        hinge pin perpendicular to the plane including the tubes;    -   means for articulating the first transverse rod about a hinge        pin perpendicular to the plane including the tubes, situated        towards the front end of the first tube and held fixed in        translation, and means for articulating the first transverse rod        about a hinge pin perpendicular to the plane including the        tubes, situated towards the rear end of the second tube and        movable in translation in a receiving seat extending along the        second tube;    -   means for articulating the second transverse rod about a hinge        pin perpendicular to the plane including the tubes, situated        towards the front end of the second tube and held fixed in        translation, and means for articulating the second transverse        rod about a hinge pin perpendicular to the plane including the        tubes, situated towards the rear end of the first tube and        movable in translation in a receiving seat extending along the        first tube;    -   and means for moving the tubes towards one another in a        controlled manner.

The means for moving the tubes towards one another can comprise areceiver articulated in rotation about a hinge pin perpendicular to theplane including the tubes and passing through the second transverse rodin proximity to the first tube, an externally threaded rod of which oneend is inserted in the receiver and the other end is inserted in theinternal space of a tube equipped with a thread corresponding to thethread of the rod, and means allowing the surgeon to turn the tube inorder to regulate the depth of insertion of the rod in the tube, and areceiving seat traversed by the rod and articulated in rotation about ahinge pin perpendicular to the plane including the tubes and passingthrough the first transverse rod in proximity to the second tube.

The instrument can comprise a spring around the threaded rod, bearing onthe receiver and the receiving seat.

According to a second variant of the invention, the interbody implantcan comprise a central part and two lateral parts, which can beconnected to the central part by connection means.

The connection means can comprise tapped holes formed in the end facesof the central part, and screws which can be inserted into the tappedholes and whose heads can come to rest on bearing surfaces formed on thelateral parts.

An instrument set for fitting an implant of the above type in place cancomprise:

-   -   a rod provided with means, at one of its ends, for fixing the        central part of the implant;    -   and two tools formed by a sheath provided at one of its ends        with means permitting one of the lateral parts of the implant to        be maintained there, and a screwdriver arranged inside the        sheath and provided with an impression which can cooperate with        the screw.

The instrument set preferably also comprises:

-   -   a crosspiece with which it is possible to maintain the rod and        the tool in determined respective angular positions during        fitting of the first of the two lateral parts of the implant,    -   a crosspiece with which it is possible to maintain the two tools        in determined respective angular positions during fitting of the        second of the lateral parts of the implant.

A tool to assist in introducing an interbody implant into a receivingseat formed in an intervertebral disc may comprise:

-   -   a first part including a protector guide of which one end,        intended to remain at the inlet of the receiving seat during the        introduction, has a width “l” substantially equivalent to the        height of the receiving seat and is equipped with stops intended        to bear against the outer surfaces of the vertebrae;    -   a second part including a distractor element placed at the end        of a rod;    -   a third part including a tubular element into which the rod of        the second part can be inserted;    -   and means permitting assembly of the three parts in a position        permitting insertion of the end of the protector guide and the        distractor element into the receiving seat, then disassembly of        the three parts in such a way as to leave only the end of the        protector guide in the receiving seat.

The means permitting assembly and disassembly of the three parts caninclude:

-   -   protrusions formed on the sides of the end of the protector        guide;    -   receiving seats formed on the upper and lower faces of the        distractor element in order to insert the protrusions of the        protector guide therein;    -   at least one stud formed on the anterior face of the tubular        element and at least one corresponding notch formed on the        posterior face of the distractor element;    -   an orifice formed on the rod of the second part, a tapped hole        formed on the tubular element of the third part, and a threaded        element which can be inserted into the tapped hole and the        orifice in such a way as to block the second and third parts        relative to one another while at the same time blocking the        protrusions of the protector guide in their receiving seats        provided on the distractor element.

As will have been understood, the invention lies in providing apossibility of interconnection between two or more interbody implantswhile or after they are/have been placed separately in the degenerateddisc (in other words in situ) in such a way as to form there a stableand integrated platform. Thus, in functional terms, a single implant isobtained which is made up of a plurality of segments connected to oneanother, without the disadvantages and risks, which would be involved inimplanting a single monobloc implant by a posterior approach.

Further objects, embodiments, forms, aspects, features, benefits, and/oradvantages will be apparent from the description and drawings providedherewith.

BRIEF DESCRIPTION OF DRAWINGS

The present invention will be better understood on reading the followingdescription in which reference is made to the attached figures:

FIG. 1 shows, viewed in perspective, of two half-implants, male (FIG.1A) and female (FIG. 1B), according to one embodiment of the presentinvention;

FIG. 2 shows, in a plan view (FIG. 2A) and in a side cross-sectionalview along 2B-2B, the two half-implants from FIG. 1 in the connectedstate;

FIG. 3 shows, viewed in perspective, a second example of twohalf-implants, male (FIG. 3A) and female (FIG. 3B), according to theinvention;

FIG. 4 shows, viewed in perspective, the two half-implants from FIG. 3in the connected state;

FIG. 5 shows, viewed in perspective, an instrument designed for thepositioning and connection of two half-implants according to theinvention;

FIG. 6 shows, in a plan view and longitudinal section, a variant of theinstrument in FIG. 5; and

FIG. 7 shows, in perspective, an instrument to assist in fitting ahalf-implant according to the invention, on the one hand in theassembled state (FIG. 7A) and on the other hand in the disassembledstate (FIGS. 7B, 7C, 7D).

FIG. 9 shows, in perspective, a rod which is used when fitting theimplant in FIG. 8;

FIG. 10 shows a tool which is also used during this fitting; and

FIGS. 11 and 12 show, in perspective, two stages of this fitting.

DESCRIPTION OF SELECTED EMBODIMENTS

For the purposes of promoting an understanding of the principles of theinvention, reference will now be made to the embodiments illustrated inthe drawings and specific language will be used to describe the same. Itwill nevertheless be understood that no limitation of the scope of theinvention is thereby intended. Any alterations and further modificationsin the illustrated device, and further applications of the principles ofthe invention as illustrated or described herein are contemplated aswould normally occur to one skilled in the art to which the inventionrelates.

To implant the connectable interbody implants according to oneembodiment of the invention by the posterior route, the surgeon begins,in a conventional manner, by partially resecting the articular facets ofthe two vertebrae concerned in the operation so as to gain access to thedegenerated disc. The surgeon then performs a partial discectomy so asto create fusion beds corresponding to the external shape of each of thehalf-implants, and the surgeon prepares the vertebral plates so as topermit subsequent fusion of the vertebrae using bone grafts enclosed bythe implants which are going to be fitted. The above-describeddiscectomy and vertebral plate preparation techniques are widely known.U.S. Pat. No. 6,174,311 issued on Jan. 16, 2001 to Branch et al., forexample, describes such techniques. In one embodiment of the presentinvention, the two cavities formed during the partial discectomy mustcommunicate with one another in the anterior region of the disc, in sucha way as to permit connection of the half-implants in the last stage oftheir fitting. In one form, this technique can be performed with toolssuch as those described in the published PCT patent applicationWO-A-0128469, which is hereby incorporated by reference. Preparing thevertebral plates and maintaining the desired intervertebral distanceduring fitting of the half-implants can be achieved with the aid of aninstrument, which will be described below. After the preparation stage,the half-implants are inserted. Two examples of pairs of suchhalf-implants will be described herein, it being understood that theseexamples are nonlimiting.

According to the first of these illustrative embodiments of theinvention, the two half-implants represented in FIG. 1, namely a malehalf-implant 1 a and a female half-implant 1 b, each include aninterbody cage 2 intended to enclose a bone graft or bone graftmaterial. Each cage 2 comprises a first part 3 intended to be orientedin the posterior-anterior direction of the spine, and a second part 4oriented substantially perpendicular to the first part 3, and beingconnected to the first part 3 via a transition portion 5. In oneembodiment, the walls of the cage 2 at the transition portion 5 have acurvilinear shape, that is to say without sharp corners which coulddamage the surrounding organs during and after fitting of thehalf-implants 1 a, 1 b. In the illustrated embodiment, the cage 2 has aset of first holes 6, 7 which pass right through it between its uppersurface 202 and its lower surface 204, and a set of second holes 8, 9which pass right through the cage 2 between its side surfaces 206 and208. These holes 6, 7, 8, 9 make it possible to insert bone graftmaterial into the cage 2, prior to the half-implants 1 a, 1 b beingfitted. Once implanted, the bone graft material in the cage 2 will comeinto contact with the vertebral plates and the remaining part of thedisc. Upon bone ingrowth into the cage 2, the bone graft material aidsin fusing the vertebrae together. Front face or tool engagement face 10of the first part 3 of each half-implant 1 a, 1 b has a hole 11 thatallows the half-implant 1 a, 1 b to be connected to an implantationtool, such as the ones which will be described below with reference toFIGS. 5 and 6. The holes 11 in one form are threaded so as to engagewith screws on the implantation tool. As depicted in the illustratedembodiment, each hole 11 has a pair of alignment notches 209 formed onopposite sides of the hole 11, which are used to orient thehalf-implants 1 a, 1 b with the implantation tool.

As shown, the two half-implants 1 a, 1 b in the first illustratedembodiment have second parts 4 with different front or connection faces12, 17. The connection face 12 of the second part 4 of the malehalf-implant 1 a includes a protrusion 13. The protrusion 13 comprisessuccessively, starting from connection face 12:

-   -   a first substantially cylindrical portion 14 having a given        diameter “D1”;    -   a second substantially cylindrical portion 15 having a diameter        “D2” that is greater than “D1”;    -   and a substantially frustoconical portion 16 whose initial        diameter is equal to “D2” and decreases as the distance from        connection face 12 increases.

The connection face 17 of the second part 4 of the female half-implant 1b includes a socket or receiver 18 for the protrusion 13 of the malehalf-implant 1 a. The socket 18 is delimited externally by a series ofelastic tongues 19. Each of the tongues 19 ends in a bulge 20 having abevelled part 21 which cooperates with the frustoconical portion 16 uponengagement of the protrusion 13 in the socket 18, in such a way as tospread the tongues 19 apart. The tongues 19 return to their initialposition after complete engagement of the protrusion 13 in the socket18. The bulges 20 then cooperate with the connection face 12 of the malehalf-implant 1 b and the cylindrical portions 14, 15 of the protrusion13 so as to maintain engagement between the protrusion 13 and the socket18. The two half-implants 1 a, 1 b are thus connected to form thefunctional equivalent of a single implant, such as is represented inFIG. 2, and they can only be disconnected by a deliberate action exertedupon their means of connection.

In the example shown, the half implants 1 a, 1 b, once connectedtogether, have a horseshoe or U-shape. The height of each half implant 1a, 1 b tapers or decreases progressively from second part 4 to the firstpart 3. In one embodiment, each half implant 1 a, 1 b tapers in amanners that generally matches the lordotic angle of the vertebrae. Itis contemplated, however, that in other embodiments the upper 202 andlower 204 surfaces of the first part 3 can extend generally in aparallel relationship and/or can be substantially cylindrical in shape.The second parts 4 of the half implants 1 a, 1 b too can have variousshapes, and the one shown in FIGS. 1 and 2 are only examples.

According to a second illustrated embodiment of the invention, twohalf-implants 22 a, 22 b represented in FIG. 3 are similar, in theirgeneral design, to those of the first embodiment shown in FIGS. 1 and 2.It should be noted that their common elements have been designated bythe same reference numbers. As shown in FIG. 3A, the male half-implant22 a includes, on connection face 12 of its second part 4, a protrusion23 which this time has an external shape that is generally cylindricaland, as is shown, defines a centrally located recess or cavity 210. Inone form, bone graft material can be packed into cavity 210 in order topromote fusion. The cylindrical-shaped protrusion 23 has a cross sectionwhose circumference extends over an angle that is at least greater than180° so as to define two receiving seats or grooves 24, 25 proximal theconnection face 12.

The female half-implant 22 b includes, on the connection face 17 of itssecond part 4, two elastic tabs 26, 27 which between them define aninternal space 28 forming a receiver or socket for accommodating theprotrusion 23 of the male half-implant 22 a and for holding it there. Inone form, notches 29, 30 are defined in the tabs 26, 27 proximal theconnection face 17 of the female-half implant 22 b, which facilitate thespreading of the tabs 26, 27 when the protrusion 23 comes into contactwith them. Ends 31, 32 of the tabs 26, 27 are shaped in such a way as tobe inserted in the receiving seats 24, 25 of the zones of connection ofthe protrusion 23 with the connection face 12 of the male half-implant22 a when the protrusion 23 is entirely engaged in the internal space 28defined by the tabs 26, 27. In this way the protrusion 23 is firmly heldin this internal space 28 and can be dislodged therefrom only by adeliberate action exerted upon the half-implants 22 a, 22 b. FIG. 4shows the two half-implants 22 a, 22 b in the assembled position formingthe functional equivalent of a single implant. As should be appreciated,the half-implants 22 a, 22 b illustrated in FIG. 4 allow for greatertolerance with respect to possible discrepancies in height between thetwo half-implants 22 a, 22 b when they are connected. A slightdiscrepancy in height between the two half-implants 22 a, 22 b does notcompromise their connection or the maintenance of their connection.

Techniques and instrumentation for implanting the half-implants 1 a, 1b, 22 a and 22 b will be described below with reference to FIGS. 5 and6. Although the technique and instrumentation for inserting thehalf-implants 1 a, 1 b, 22 a and 22 b will be described below withreference to half-implants 1 a and 2 b, it should be understood that thebelow described techniques and instrumentation can be used withhalf-implants 22 a and 22 b as well as other types of half-implants.Before the half-implants 1 a, 1 b are inserted, the disc space isprepared in the manner as described above. The insertion of the of thehalf-implants 1 a, 1 b is then performed by attaching the half-implants1 a, 1 b to the end of rods that are inserted in the holes 11 of thefront faces 10 of their first parts 3, and by connecting thehalf-implants 1 a, 1 b together by manipulation of these rods. The endsof the second parts 4 of the half-implants 1 a, 1 b are then movedtowards one another in order to connect them to one another in situ.Instrument 33 and 33′ (implantation tool) allow for the connection ofthe half-implants 1 a, 1 b in such a manner. The instrument 33, 33′allows the half-implants 1 a, 1 b to be moved towards each other in agradual and controlled manner while constantly keeping them in aparallel orientation. This improves the chance that the insertion of theprotrusion 13, 23 of the male element 1 a, 22 a in the correspondingreceiving seat 18, 28 of the female element 1 b, 22 b is effected underoptimum conditions.

FIG. 5 illustrates one embodiment for the instrumentation 33 that isused to implant the half-implants 1 a, 1 b, and FIG. 6 illustratesinstrumentation 33′ according to another embodiment. It should be notedthat instrumentation 33 and 33′ share a number of common features. Forthe sake of brevity and clarity, instrumentation 33 and 33′ will bediscussed together below with particular reference to instrumentation33. Although instrumentation 33 will be referenced below, it should beunderstood that the same description for instrumentation 33 applies aswell as to instrumentation 33, with any notable distinctions between thetwo embodiments being highlighted. With reference to the embodimentillustrated in FIG. 5, the instrumentation 33 includes two rods 34, 35composed of several parts that are connected together by systems which,for example, can include slots or holes and fixed or detachable indexingpins. In the FIG. 5 embodiment, the parts of the rods 34, 35 areconnected together with detachable indexing pins 36, 37. It iscontemplated, however, that the rods 34, 35 can each be made from asingle part. For example, in the embodiment illustrated in FIG. 6, theinstrumentation 33′ includes two rods 34′, 35′, each having a unitaryconstruction.

The rods 34, 35 are intended to support the half-implants 1 a, 1 b, whenthe half implants 1 a, 1 b are connected together. However, the rods 34,35 can also be used alone and independently by the surgeon formanipulating the half-implants 1 a, 1 b during insertion of thehalf-implants 1 a, 1 b in the receiving cavities (or seats) hollowed outin the degenerated disc. To fix the half-implants 1 a, 1 b in theillustrated embodiment, the rods 34, 35 each have a bent end part 38, 39that defines an orifice 40, 41 in which a screw 42, 43 is received. Inthe embodiment illustrated in FIG. 5, these bent parts 38, 39 have askewed shape in which the bent parts 38, 39 bend away from a planedefined by the rods 34, 35. By skewing bent parts 38 and 39 in such amanner, the area around the orifices 40, 41 is freed so that the screws42, 43 can be easily accessed. However, it should be appreciated thatthe bent parts 38, 39 can have a planar arrangement. For instance, inthe embodiment illustrated in FIG. 6, bent parts 38′ and 39′ areoriented in the same plane as the rest of the rods 34′, 35′. At theextremity of the end part 38, 39, each rod 34, 35 has tabs 44, 45, 46,47 intended to be inserted in receiving seats or alignment notches 209formed on the sides of the holes 11 provided on the front faces 10 ofthe half-implants 1 a, 1 b, or in separate receiving seats speciallydesigned for this purpose. These tabs 44, 45, 46, 47 make it possible tofix the half-implants 1 a, 1 b on the rods 34, 35 in definedorientations. The screws 42, 43, cooperating with the threads formed onthe surfaces delimiting the holes 11 in the half-implants 1 a, 1 b,permit this fixation. The bent portions 38, 39 of the rods 34, 35 permitaccess to heads 212 of the screws 42, 43 so that these can be tightenedand loosened. It goes without saying that the device for fixing thehalf-implants 1 a, 1 b which has just been described is but one exampleand that the skilled person can imagine other examples which wouldsatisfy the same functions.

After the half-implants 1 a, 1 b have been secured to the rods 34, 35,the rods 34, 35 are introduced into a positioning device 214 that allowsthem to be held in a position in which the rods 34, 35 are orientedparallel with respect to one another. The positioning device 214 alsomakes it possible to regulate the spacing of the rods 34, 35 from eachother so that the surgeon can bring the half-implants 1 a, 1 b towardseach other and connect the half-implants 1 a, 1 b, after thehalf-implants 1 a, 1 b have been fitted in the receiving cavitieshollowed out in the disc. In the example shown in FIGS. 5 and 6, thepositioning device 214 in the first instance includes two tubes 48, 49in which the rods 34, 35 are inserted. The rods 34, 35 are secured inthe tubes 48, 49 by stop members or bearings 50, 51 (or any otherfunctionally equivalent device) formed on the rods 34, 35 on which thetubes 48, 49 come into abutment, and by internally threaded rings 52, 53which, by cooperating with corresponding threads 216 formed on the endsof the rods 34, 35, press the stop members 50, 51 against the tubes 48,49.

The tubes 48, 49 are connected to one another by means of an articulateddevice 218. The articulated device 218 in the first instance includestwo transverse rods 54, 55, which are pivotally coupled to one anotherto permit their rotation about a common hinge pin 56, which extendsperpendicular to the plane that includes the tubes 48, 49. The firsttransverse rod 54 is pivotally coupled to the distal or front end of thefirst tube 48 (that is to say the end nearest the half-implant 1 a)about a hinge pin 57 that extends parallel to the common hinge pin 56 ofthe transverse rods 54, 55. Hinge pin 57 is held in a receiving seat 58which is fixed to the first tube 48 and which prohibits any movementthereof in translation relative to the first tube 48. The pivoting ofthe first transverse rod 54 about the hinge pin 57 is ensured by astirrup 59. As shown, the stirrup 54 of the first transverse rod 54extends around opposite sides of tube 48. The first transverse rod 54 isalso coupled to the proximal or rear end of the second tube 49 (that isto say the end farthest from the half-implant 1 b) about a hinge pin 60.A knurled wheel 61 is threadedly coupled to hinge pin 60. Hinge pin 60extends in parallel relationship with respect to the common hinge pin 56of the transverse rods 54, 55. Hinge pin 60 is coupled with the firsttransverse rod 54 by a stirrup 62, which can pivot about hinge pin 60.Hinge pin 60 is able translationally move in a receiving seat or slot 63that extends along the second tube 49. The second transverse rod 55 issimilarly pivotally coupled to the front end of the second tube 49 abouta hinge pin 64. Hinge pin 64 is held in a receiving seat 65 fixed to thesecond tube 49. As shown, rod 55 has a stirrup 66 that is receivedaround tube 49, and the stirrup 66 is coupled to hinge pin 64. Thesecond transverse rod 55 is also pivotally coupled to the rear end ofthe first tube 48 with a hinge pin 67. A knurled wheel 68 is threadedlycoupled to the hinge pin 67. Hinge pin 67 is coupled to the secondtransverse rod 55 by a stirrup 69, which can pivot about hinge pin 67.Hinge pin 67 is able to translationally move in a receiving seat or slot70 that extends along the first tube 48. The use of knurled wheels 61,68 and the threaded hinge pins 60, 67 cooperating with correspondingthreads formed in the stirrups 62, 69 ensures simple assembly anddisassembly of the device 214.

Device 214 includes a receiver 71 that rotates about a hinge pin 72 thatextends parallel to the other axes of rotation for hinge pins 56, 57,60, 64 and 67, which were mentioned above. Hinge pin 72 passes throughthe second transverse rod 55 in proximity to the first tube 48. One endof an externally threaded rod 73 is inserted and fixed in receiver 71.The other end of the threaded rod 73 is inserted in the internal spaceof a tube 74 equipped with a thread corresponding to the thread of therod 73. Tube 74 is equipped with a knurled wheel 75 that allows thesurgeon to turn it and thereby regulate the position of the rod 73relative to the tube 74. As shown, a receiving seat 76 is rotatablycoupled a hinge pin 77 that passes through the first transverse rod 54in proximity to the second tube 49. Receiving seat 76 is able to rotateabout an axis that is extends parallel to the axes of rotation of hingepins 56, 57, 60, 54, 67 and 72, which were mentioned above. The threadedrod 73 passes through receiving seat 76.

After the whole instrument 214 has been assembled, the surgeon is ableto move the two rods 34, 35 supporting the half-implants 1 a, 1 btowards or away from each other by turning the knurled wheel 75 in sucha way as to regulate the penetration depth of the rod 73 in the tube 74,which determines the spacing of the rods 34, 35. Also, the ability ofthe transverse rods 54, 55 to rotate about the various hinge pins 56,57, 60, 64, 67, 72, 77, and the ability of translational movement ofhinge pins 60 and 67 in receiving slots 63 and 70, mean that thismovement of the rods 34, 35 towards or away from each other is effectedwhile ensuring permanent parallel positioning of the rods 34, 35. Thehalf-implants 1 a, 1 b can therefore be moved towards each other in thedirection most favourable to their proper connection.

A spring 78 is provided about the threaded rod 73 and bears on receiver71 and receiving seat 76. The spring 78 makes it possible to increasethe rigidity of the assembled instrument because the spring 78 tends tospace the rods 34, 35 apart from one another in order to reduce the playwhich may exist between the different components controlling the spacingof the rods 34, 35. Reduction of this play can make use of theinstrument 214 easier.

One should appreciate that variations can be made to the design of thepositioning device 214 for controlling the movement of the two rods 34,35. For example, it would be conceivable to move the rods 34, 35together using an elastic device which can be controlled by a clamp orby simple manual pressure. The threaded-tube and threaded-rod devicewhich has been described has the advantage of allowing easily controlledprogressive clamping of the rods 34, 35, and permanent holding of aselected relative position of the rods 34, 35, permitting, if necessary,the half-implants 1 a, 1 b to move towards each other in several stageswithout requiring the surgeon to manually holds the rods 34, 35 in orderto keep the spacing of the rods 34, 35 constant.

Once the half-implants 1 a, 1 b have been connected to one another, therings 52, 53 are removed so as to allow disconnection of the rods 34, 35from the rest of the positioning device 214. The screws 42, 43 are thenloosened so as to disconnect the rods 34, 35 from the half-implants 1 a,1 b whose implantation is then complete.

According to another variation of the present invention, the interbodyimplant no longer comprises two parts, but three parts, namely a centralpart and two lateral parts which are fixed together more or less rigidlydepending on the wishes of the user. An illustrative embodiment of thisvariation is shown in FIGS. 8A and 8B. In implant assembly 224, acentral part 106 of the implant 224 is in the form of a cage, which hasapertures 107 through its upper and lower faces and apertures 108through its lateral faces, in such a way as to permit a bone graftmaterial to be inserted into the central part 106 prior to its fitting.The central part 106 has a general shape that is slightly arched inorder to match the shape of the receiving cavity that has been formed inthe disc. The ends of the central part 106 include end faces 109, 110oriented obliquely in relation to the general direction of the centralpart 106, so as to be easily accessible from the outside when thecentral part 106 is in place in its receiving cavity. The implant 224also includes two lateral parts 111, 112 in the form of cages, whichhave apertures 113, 114, 115 and 116 for receiving bone graft material.The lateral parts 111, 112 are connected to the central part 106 by wayof screws 117, 118, which can be inserted into tapped holes 119 formedin the end faces 109, 110 of the central part 106, and the heads ofwhich engage bearing surfaces 120 formed on the lateral parts 111, 112.This assembling of implant 224 by means of screws 117, 118 gives itgreat rigidity.

To fit implant 224 in place, the following described techniques can beused. The central part 106 is fixed with the aid of a screw 121 (which,as is shown, can be identical to one of the screws 117, 118 which willbe used for assembling the implant 224) to the end of a rod 122 which isdesigned, for example, in a manner similar to one of the rods 34, 35 ofthe instrument set shown in FIGS. 5 and 6, as can be seen from FIG. 9.The surgeon then introduces the central part 106 into the receivingcavity that was formed beforehand in the damaged disc. Of course, itshould be appreciated that other means of fixing other than the screw121 are conceivable.

In the following stage, the surgeon uses a tool 123, shown in FIG. 10,which is made up of two main parts:

-   -   a sheath 124 provided at one of its ends with means permitting        one of the lateral parts 111, 112 of the implant to be        maintained there, for example comprising two studs or tabs 125        which cooperate with two corresponding notches 126, 127 formed        on the anterior face 128 of each of the lateral parts 111, 112;        and    -   a screwdriver 129 extending inside and through the sheath 124;        one of its ends is provided with an impression or head 130 which        can engage with the screw 117, and the other end is provided        with a grip handle 131 that allows the surgeon to turn the        screwdriver 129.

In the example shown, a tube 132 is positioned inside the sheath 124such that there is only a slight clearance between the tube 132 and thesheath 124 so that the tube 132 is able to move inside the sheath 124.As depicted in FIG. 10, the tube 132 in the sheath 124 has externallythreaded end 226, and the tube 132 can be rotated through a knurledwheel 132′, which is attached to the tube 132. When rotated, thethreaded end 226 of the tube 132 is used to engage with a correspondingthreading formed in a bore 137 of the anterior face 128 of a lateralpart 111, 112 of the implant 224. One could of course conceive of othermeans for maintaining the lateral part 111, 112 of the implant 224connected to the sheath 124. As shown, the screwdriver 129 is slidablyreceived inside tube 132 such that the screwdriver 129 is able to rotateinside tube 132 and tighten screw 117.

Referring to FIG. 11, lateral part 111 of the implant 224 is secured tothe threaded end 226 of the tube 132 in tool 123, and the screw 117 isplaced at the end of the screwdriver 129 of the tool 123. Lateral part111 of the implant 224 is then inserted into the receiving cavity formedin the disc in order to move it to the central part 106, which is stillfixed to the rod 122. Lateral part 111 is positioned to bring the screw117 in alignment with the tapped hole 119 formed in the face 109 of thecentral part 106. To facilitate this positioning, in one embodiment, acalibrated crosspiece 133 is used to maintain the rod 122 and the tool123 in a suitable angular orientation. The crosspiece 133 also makes itpossible to obtain a precise placement of the central part 106 of theimplant in its receiving cavity. When the desired positioning isobtained, the surgeon turns the screwdriver 129 in order to tighten thescrew 117 so as to secure the central part 106 and the lateral part 111.FIG. 11 illustrates the configuration of the implant assembly 224 at theend of this stage (the disc and its surrounding area have not been shownfor the sake of clarity).

After the lateral part 117 is attached to the central part 106, the rod122 is then disconnected from the central part 106 of the implant 224 byloosening the screw 121. The other lateral part 112 of the implant 224is attached at the threaded end 226 of a tool 123′, which is identicalto the previous tool 123. Screw 118 is placed at the end of thescrewdriver 129′ in the tool 123′. With tool 123′, lateral part 112 ismoved to engage the end face 110 of the central part 106 that wasdisengaged from rod 122. The other lateral part 112 is then secured tothe central part 106 with screw 118 in the same manner as describedabove. In one embodiment, a crosspiece 134 is used to maintain theangular positioning of the two tools 123, 123′. FIG. 12 shows theassembly 224 at the end of this stage. Once the other lateral part 112has been fixed, the two tools 123, 123′ are withdrawn by rotation of theknurled wheels 132′ making it possible to detach the tubes 132 from thebores 137 of the anterior faces 128 of the lateral parts 111, 112.

When the two tools 123, 123′ are removed, the insertion of the implant224 is thus completed. In one embodiment, to make it easier to align thescrews 117, 118 with the tapped holes 119 of the central part 106, ametal wire is passed through a central aperture 135 (FIG. 8B) in thescrew 121 that connects rod 122 to the end face 109 of the central part106. The wire is further passed through the tapped hole 119 of the otherend face 110 of the central part 106. The screws 117, 118 and thescrewdrivers 129, 129′ are themselves provided with central apertures135, 136, 136′ in which the wire can be passed through. With the wirepassing through the screws 117, 118 and the screwdrivers 129, 129′, theend of the portion of the wire extends from tapped hole 119 so that thescrews 117, 118 are correctly guided towards the tapped holes 119. Afterthe rod 122 has been withdrawn, the other end of the wire emerging fromthe central part 106 becomes accessible, and the screw 118 carried bythe screwdriver 129′ of the tool 123′ can be engaged on this end of thewire. After the implant 224 has been fitted, the wire is removed.

The above-described embodiment in which the implant includes three partshas at least the following advantages:

-   -   at each operation, only a component of relatively small        dimensions is inserted into the body of the patient; this        entails a less invasive approach; it is no longer necessary to        remove as many stabilizing elements like the articular facets of        the vertebrae, and it is possible to dispense with sectioning        the muscles, and instead they need only be moved aside: recovery        is therefore quicker; and    -   the connection operations take place closer to the openings of        the cavity formed in the disc, thus in a zone which is visible        to the surgeon, making it easier to fit the implant; moreover,        this permits a connection by screws, thus one which is rigid,        reliable and relatively easy to effect.

The connection means which have been described and shown for thisillustrative embodiment of the invention are not exclusive, and theperson skilled in the art will be able to adopt other connection meansanalogous, for example, to those described for the previously describedimplant variants made up of two parts. The degree of rigidity sought forthe assembled implant will be a criterion in choosing the method ofconnecting the different parts. The person skilled in the art will bereadily able to adapt the configurations of the different parts of theimplant to this effect, and of the tools used to fit them, by drawinginspiration from what has been set out in this description.

When the implants (1 a, 1 b), (22 a, 22 b), (106, 111, 112) are beingput in place, it is possible for the surgeon to use spacer tool 78,which aids in introducing the parts of the implants (1 a, 1 b), (22 a,22 b), (106, 111, 112) into their respective receiving cavities. Tool 78maintains the space between the vertebrae concerned, while at the sametime protects the surrounding area, in particular the spinal nerves,which is very important in the posterior lumbar region where theimplants (1 a, 1 b), (22 a, 22 b), (106, 111, 112), according to oneembodiment, are intended to be implanted.

This tool 78 is made up of three parts. The first part is a protectorguide 79 of which one end 80 is intended to be positioned at the inletof one of the receiving cavities formed in the vertebrae for thehalf-implants (1 a, 1 b), (22 a, 22 b), (106, 111, 112) during theirintroduction. The rest of tool 78 is designed to protect the surroundingarea of the work zone and to permit easy manipulation by the surgeon,for example, as is shown, in the form of a straight part finishing in abent part 82 set outwards from the operating site. As shown in FIG. 7B,end 80 of tool 78 has a width “l” largely equivalent to the height ofthe receiving cavities formed in the disc for fitting of the implants (1a, 1 b), (22 a, 22 b), (106, 111, 112). It is equipped with stops 81, 82that are configured to bear against the outer surfaces of the vertebraeupon introduction of the instrument into the receiving cavity, so as tolimit its penetration to the necessary length. End 80 of the protectorguide 79 is also equipped on its sides with two protrusions 83, 84 thathave a generally triangular shape and are equipped with posteriornotches 85, 86 whose function will be explained below.

The second part of tool 78 is a distractor element 87, which isconfigured to support the end 80 of the protector guide 79 upon itsinsertion into the receiving cavity formed in the vertebrae for theimplant (1 a, 1 b), (22 a, 22 b), (106, 111, 112). The distractorelement 87 is tapered at its front end 88 to facilitate its introductioninto the receiving cavity formed in the vertebrae. The distractorelement 87 is placed at the end of a rod 89 which has, on itscylindrical lateral wall, an orifice 90 whose function will be explainedbelow. Engagement seats are formed in the upper face 91 and lower face220 of the distractor element 87 for the protrusions 83, 84 of theprotector guide 79. These engagement seats are each delimited on the onehand by a stud 92 which is received in one of the posterior notches 85,86 in the protrusions 83, 84 of the and on the other hand by ahollowed-out portion 93 which receives the free corner 94, 95 of one ofthe protrusions 83, 84 by blocking its movements towards the front ofthe distractor element 87. The distractor element 87 also has, on itsrear face, two notches 96 whose function will be explained below.

The third part of tool 78 is a tubular element 97, with rectangularcross-section in the example shown, and provided with a receiving cavity98 permitting insertion of the rod 89 of the second part of the tool 78.One of the lateral surfaces 222 of the tubular element 97 has a tappedhole 99 in which a threaded element 100 with a knurled wheel 101 isthreaded. End 102 of the threaded element is configured to penetrateinto the orifice 90 of the rod 89 of the distractor element 87 when tool78 is assembled. The position of the tapped hole 99 is determinedaccordingly. Anterior face 103 of the tubular element 97 has studs 104,105 that are configured to be inserted in the notches 94 of theposterior face of the distractor element 87. The studs 104, 105 and thenotches 94 make it possible to regulate the relative positions of thedistractor element 87 and the tubular element 97 in such a way thatorifice 90 in the distractor element 87 and the tapped hole 99 in thetubular element 97 are automatically aligned with one another uponassembly of the tool 78. It would of course be possible to provide justone stud 102, 103 and a single notch 94 or any other means ofestablishing suitable relative positions of the distractor element 87and the tubular element 97 when assembling the tool 78.

Tool 78 is assembled as follows. In a first stage, the protector guide79 and the distractor element 87 are placed one on top of the other,with the triangular protrusions 83, 84 of the protector guide 79inserted in the corresponding receiving seats of the distractor element87. In a second stage, the rod 89 of the distractor element 87 isinserted in the receiving cavity 98 of the tubular element 97 andsecured inside the receiving cavity 98 with the threaded element 100,which is made to penetrate into orifice 90. The triangular protrusions83, 84 are thus secured in their respective receiving seats on thedistractor element 87 by the anterior face 103 of the tubular element97, and the whole tool is thus stabilized, with the protector guide 79held along the tubular element 97. The distractor element 87 and the end80 of the protector guide 79 are then inserted in a receiving cavitypreviously hollowed out in the degenerated disc, in such a way as toestablish the exact interdiscal distance necessary for introducing thecorresponding implant (1 a, 1 b), (22 a, 22 b), (106, 111, 112), whichdistance is substantially equal to “l”, and to complete the preparationof the receiving seat surfaces. The threaded element 100 is thenloosened, which allows the third part of the tool to be withdrawn, thenthe second part, so that only the end 80 of the protector guide 79 isleft remaining in the receiving cavity. A part of implant (1 a, 1 b),(22 a, 22 b), (106, 111, 112) is then inserted in the receiving cavity.With the end 80, it is possible to maintain the desired interdiscalspace during this introduction, while the rest of the protector guide79, which extends outside the receiving cavity for the implant (1 a, 1b), (22 a, 22 b), (106, 111, 112), makes it possible to move aside andprotect the organs of the patient which are situated on the path ofinsertion of the implant (1 a, 1 b), (22 a, 22 b), (106, 111, 112).

It goes without saying that detailed modifications can be made to thevarious parts of the tool 78 (in particular as regards the means ofjoining its various parts), provided that the essential functions of itsvarious elements are retained. This tool can also be used to assist inplacing interbody implants having a configuration different than that ofthe implants (1 a, 1 b), (22 a, 22 b), (106, 111, 112) described herein.

While the invention has been illustrated and described in detail in thedrawings and foregoing description, the same is to be considered asillustrative and not restrictive in character, it being understood thatonly the preferred embodiment has been shown and described and that allchanges, modifications, and equivalents that come within the spirit ofthe inventions described herein and/or defined by the following claimsare desired to be protected.

1. Intersomatic implant for inserting and maintaining a bone graft inplace in a receiving seat formed in a disc with a view to obtainingintervertebral fusion, comprising: a plurality of interbody cages, eachof said interbody cages defining a substantially open hollow interiorand including upper and lower vertebral engaging surfaces and an openingpassing through said upper and lower vertebral engaging surfaces andinto communication with said hollow interior to facilitate fusion withadjacent vertebrae; and means for in situ connection of two consecutiveones of said interbody cages, and in that the implant as a wholecomprises a horseshoe-shaped or U-shaped implant assembly having twogenerally linear lateral portions extending substantially parallel toone another in an anterior-posterior direction and a central portiondefining a generally semi-circular curvature extending between saidgenerally linear lateral portions.
 2. Intersomatic implant according toclaim 1, characterized in that each of said two consecutive ones of saidinterbody cages includes either a protrusion, making this implant a maleimplant, or a receiver for inserting and holding said protrusion, makingthis implant a female implant, in order to achieve and maintain aconnection between said male implant and said female implant.
 3. Theintersomatic implant according to claim 2 wherein the protrusion of themale implant successively comprises; a first substantially cylindricalportion having a given diameter “D1”; a second substantially cylindricalportion having a diameter “D2” greater than “D1”; and a substantiallyfrustoconical portion whose initial diameter is equal to “D2” anddecreases toward a distal end of the protrusion.
 4. The intersomaticimplant according to claim 3, wherein: the receiver of the femaleimplant is delimited externally by elastic tongues which finish in abulge; and the shape and the dimensions of the tongues of the femaleimplant and of their bulges making them able to cooperate with theprotrusion of the male implant in order to achieve and maintain aconnection of the male and female implants.
 5. The intersomatic implantaccording to claim 2, wherein the receiver of the female implantincludes a plurality of elastic tongues, each of said elastic tonguesincluding an end defining an inwardly extending bulge.
 6. Theintersomatic implant according to claim 2, wherein the protrusion of themale implant has a general external shape of a cylinder portion whosecross section has a circumference which extends over an angle greaterthan 180° in such a way as to define two receiving seats.
 7. Theintersomatic implant according to claim 6, wherein: the female implantincludes two elastic tabs which between them define said receiver; andthe shape and the dimensions of the elastic tabs of the female implantmaking them able to permit insertion of the protrusion of the maleimplant into the receiver, the ends of said elastic tabs being insertedinto the receiving seats in order to achieve and maintain a connectionof the male and female implants.
 8. The intersomatic implant accordingto claim 2, wherein the female implant includes two elastic tabs whichbetween them define said receiver.
 9. The intersomatic implant accordingto claim 1, further comprising bone graft material positioned withineach of said interbody cages to further facilitate fusion with theadjacent vertebrae.
 10. The intersomatic implant according to claim 1,wherein each of said consecutive ones of said interbody cages includesone of said lateral portions of said implant assembly and also includesan arc-shaped portion extending along approximately one-quarter of acircle, said arc-shaped portions of said interbody cages interconnectedto one another by said means for in situ connection to define saidsemi-circular curvature of said central portion of said implantassembly.
 11. An interbody implant for implanting between adjacentvertebrae, comprising: a male implant having an engagement protrusionextending therefrom; a female implant defining a socket; and whereinsaid male implant comprises a first interbody cage and said femaleimplant comprises a second interbody cage, each of said first and secondinterbody cages defining a substantially open hollow interior andincluding upper and lower vertebral engaging surfaces and an openingpassing through said upper and lower vertebral engaging surfaces andinto communication with said hollow interior to facilitate fusion withthe adjacent vertebrae; and wherein said protrusion of said male implantis engaged in said socket of said female implant; and wherein theinterbody implant comprises a horseshoe-shaped or U-shaped implantassembly having two generally linear lateral portions extendingsubstantially parallel to one another in an anterior-posterior directionand a central portion defining a generally semi-circular curvatureextending between said generally linear lateral portions.
 12. Theimplant of claim 11, wherein said socket is defined by a plurality ofelastic tongues.
 13. The implant of claim 11, wherein said protrusionhas a cylindrical shape and said socket has a cylindrical shape.
 14. Theimplant of claim 11, further comprising bone graft material positionedwithin each of said first and second interbody cages to furtherfacilitate fusion with the adjacent vertebrae.
 15. The implant of claim11, wherein each of said first and second interbody cages includes oneof said lateral portions of said implant assembly and also includes anarc-shaped portion extending along approximately one-quarter of acircle, said arc-shaped portions of said interbody cages interconnectedto one another by engagement of said protrusion in said socket to definesaid semi-circular curvature of said central portion of said implantassembly.
 16. Intersomatic implant for inserting and maintaining a bonegraft in place in a receiving seat formed in a disc with a view toobtaining intervertebral fusion, comprising: a plurality of interbodycages, each of said interbody cages defining a substantially open hollowinterior and including upper and lower vertebral engaging surfaces andan opening passing through said upper and lower vertebral engagingsurfaces and into communication with said hollow interior to facilitatefusion with adjacent vertebrae; and means for in situ connection of twoconsecutive ones of said interbody cages, and in that the implant as awhole defines a horseshoe-shaped or U-shaped implant assembly; andwherein the implant assembly includes a first of said interbody cagesand a second of said interbody cages, each of said first and secondinterbody cages including a lateral portion oriented in aposterior-anterior direction and an arc-shaped portion extending alongapproximately one-quarter of a circle, said arc-shaped portion of saidfirst interbody cage connected to said arc-shaped portion of said secondinterbody cage by said means for in situ connection to provide saidhorseshoe-shaped or U-shaped implant assembly.
 17. An interbody implantfor implanting between adjacent vertebrae, comprising: a male implanthaving an engagement protrusion extending therefrom; a female implantdefining a socket; and wherein said male implant comprises a firstinterbody cage and said female implant comprises a second interbodycage, each of said first and second interbody cages defining asubstantially open hollow interior and including upper and lowervertebral engaging surfaces and an opening passing through said upperand lower vertebral engaging surfaces and into communication with saidhollow interior to facilitate fusion with the adjacent vertebrae; andwherein said protrusion of said male implant is engaged in said socketof said female implant; and wherein each of said first and secondinterbody cages includes a a lateral portion oriented in aposterior-anterior direction and an arc-shaped portion extending alongapproximately one-quarter of a circle, said arc-shaped portion of saidfirst interbody cage connected to said arc-shaped portion of said secondinterbody cage by engagement of said protrusion of said male implant insaid socket of said female implant; and wherein said first and secondinterbody cages are interconnected to provide a U-shaped implantassembly.
 18. An interbody implant for implantation between adjacentvertebrae, comprising: a first interbody cage having a protrusionextending therefrom; a second interbody cage defining a socket; saidfirst and second interbody cages each defining a substantially openhollow interior and each including upper and lower vertebral engagingsurfaces that each define an opening in communication with said hollowinterior to facilitate fusion with the adjacent vertebrae; and whereinsaid protrusion of said first interbody cage is engaged in said socketof said second interbody cage to interconnect said first and secondinterbody cages; and wherein the interbody implant comprises ahorseshoe-shaped or U-shaped implant assembly having two generallylinear lateral portions extending substantially parallel to one anotherin an anterior-posterior direction and a central portion defining agenerally semi-circular curvature extending between said generallylinear lateral portions.
 19. The implant of claim 18, further comprisingbone graft material positioned within said hollow interior of said firstand second interbody cages to further facilitate fusion with theadjacent vertebrae.
 20. The implant of claim 18, wherein each of saidfirst and second interbody cages includes opposite side surfaces and anopening passing therethrough between said opposite side surfaces. 21.The implant of claim 18, wherein said socket is defined by a pluralityof elastic tongues that are engaged about said protrusion to maintainsaid first interbody cage in engagement with said second interbody cage.22. The implant of claim 18, wherein said first and second interbodycages each have a height between said upper and lower surfaces thatprogressively tapers to generally match a lordotic angle between theadjacent vertebrae.
 23. The implant of claim 18, wherein each of saidfirst and second interbody cages includes a front face defining a holefor connection with an implantation tool.
 24. The implant of claim 18,wherein each of said first and second interbody cages includes one ofsaid lateral portions of said implant assembly and also includes anarc-shaped portion extending along approximately one-quarter of acircle, said arc-shaped portions of said interbody cages interconnectedto one another by engagement of said protrusion in said socket to definesaid semi-circular curvature of said central portion of said implantassembly.
 25. An interbody implant for implantation between adjacentvertebrae, comprising: a first interbody cage having a protrusionextending therefrom; a second interbody cage defining a socket; saidfirst and second interbody cages each defining a substantially openhollow interior and each including upper and lower vertebral engagingsurfaces that each define an opening in communication with a said hollowinterior to facilitate fusion with the adjacent vertebrae; and whereinsaid protrusion of said first interbody cage is engaged in said socketof said second interbody cage to interconnect said first and secondinterbody cages; and wherein each of said first and second interbodycages includes a a lateral portion oriented in a posterior-anteriordirection and an arc-shaped portion extending along approximatelyone-quarter of a circle, said arc-shaped portion of said first interbodycage connected to said arc-shaped portion of said second interbody cageby engagement of said protrusion of said first interbody cage in saidsocket of said second interbody cage; and wherein said first and secondinterbody cages are interconnected to define a U-shaped implantassembly.